In U.S. Pat. No. 3,751,242 there has been described a process for making chromium alloys, specifically ferritechromium steel, by refining a pig-iron melt in a converter and then admixing therewith a ferrochrome of high carbon content whereupon the mixture is subjected to another oxygen blow in order to reduce the carbon content of the resulting alloy.
In commonly owned U.S. patent application Ser. No. 527,826, filed by some of us jointly with several others on Nov. 27, 1974, there has been disclosed a process for the rapid decarburization of ferro-alloys -- specifically ferrochrome and ferromanganese -- which are not admixed with a prerefined, larger mass of pig iron, that process enabling the lowering of the carbon content of the ferro-alloy at a rate of substantially 0.2% to 1% per minute. Essentially, the process of that commonly owned application involves the use of an oxygen-blowing rate of about 15 cubic meters S.T.P. per minute for each metric ton of ferro-alloy, the minimum rate being thus 3 m.sup.3 /min per ton for a decarburization rate of 0.2% C per minute. The amount of 15 m.sup.3 S.T.P. per ton-percent may be regarded as a quasi-stoichiometric equivalent.
It has been determined that, with observation of those parameters, the reactions otherwise occurring in the supernatant slag take place directly in the melt, with little formation of chromium or manganese oxides from which these nonferrous metals can be recovered only in laborious ways and at high cost. Still, the rapid decarburization described in that prior application -- with removal of 1% carbon in not more than 5 minutes -- requires a rather precise control of the oxygen feed. This creates some technical problems, especially since the relatively short reaction time makes it difficult to readjust the feed rate during the blow on the basis of readily observable parameters. Another problem encountered, especially with large quantities of ferro-alloy to be refined, is the danger of a rise in the bath temperature above levels that can be safely tolerated by commercially available converters.